Resolution of media access contentions in networks comprising a plurality of network nodes and at least one star mode

ABSTRACT

The invention relates to a network comprising a plurality of network nodes. At least part of the network nodes are directly coupled to each other via at least one star node. The star node comprises a plurality of star interfaces which are assigned to at least one network node. In dependence on a respective pilot signal, a star interface conveys a message from the assigned network node to the other star interfaces, or from another star interface to at least one of the assigned network nodes. Also in the event of simultaneous arrival of at least two pilot signals at the respective star interfaces, a decision circuit releases one star interface for the transmission of data.

SUMMARY OF INVENTION

The invention relates to a network comprising a plurality of networknodes. Such network nodes may be used, for example, in motor vehicles,in aerotechnics and aerospace engineering, in industrial automation (forexample, sensor systems) and domestic automation (e.g. lightingtechnology, alarm systems, central heatings, climatic control etc.).

In such a network for motor technology, for example, the TTP protocol(TTP=Time-Triggered Protocol) from the journal “Elektronik” no. 14,1999, pp. 36 to 43 (Dr. Stefan Polenda, Georg Kroiss: “TTP: “Drive byWire” in greifbarer Nähe”) may be used. This protocol enables a reliabledata transmission and may therefore also be used in networks forsafety-relevant devices (for example brakes). In the article mentionedabove, a bus system is mentioned as a network structure.

It is an object of the invention to provide another network comprising aplurality of network nodes.

The object is achieved by a network of the type defined in the openingparagraph,

-   in that at least part of the network nodes are directly coupled to    each other via at least one star node,-   in that the star node includes a plurality of star interfaces which    are assigned to at least one network node,-   in that one star interface transfers data from the assigned network    node to the other star interfaces or from another star interface to    at least one of the assigned network nodes each time in dependence    on a pilot signal, and-   in that also in the event of simultaneous arrival of at least two    pilot signals at the respective star interfaces, a decision circuit    releases one star interface for the transmission of data.

The idea on which this invention is based is the control of a star nodeby means of a pilot signal, which is generated by the network nodes. Thepilot signal is modified before and after a transmission of a message,so that a star interface included in the star node and assigned to anetwork node recognizes when the assigned network node changes the pilotsignal. In the star node the other star interfaces are then switched sothat they receive only one message from the star interface that receivesa message from the assigned network node.

Such a signaling is specifically suitable for time-controlledcommunication protocols, which guarantee via a respective access method(TDMA Time-Division Multiple Access) that only one respective networknode can utilize the medium for transmitting its message. However, withsuch a method, the eventual access by only one network node cannot beguaranteed. The situation of a simultaneous access may occur both in theevent of an error and during the start of the system i.e. in atransition phase, when the fixed time window is not predefined at oneand the same time for all the nodes.

According to the invention, such an access contention is solved with theaid of a decision circuit, which, when a plurality of pilot signalsarrive at the same time, releases a certain star interface fortransmitting data. The resolution of the contention within the star nodeenables the use of the network also for protocols which, for example,for a decentralized system start, depend on a contention treatment.Particularly for applications that require the highest standards ofreliability and availability from the communication system, each networknode must be in a position, by predefining the communication frame, tocorrectly resume the exchange of messages with a system start or afterthe communication during operation has failed.

As described in patent claim 2, a certain, periodically repetitive timeslot is assigned to each network node in the network, for transmittingits messages. The pilot signal generator in a network node signals thistime slot when a pilot signal is transmitted. The pilot signal may bedistinguished from the message per se in various ways. For example, thepilot signal uses a different frequency range from the signal having themessage to be transmitted.

Patent claim 3 relates to a pilot signal evaluation circuit, whichgenerates a send control signal which is activated when a pilot signalhas been sent by the assigned network node and when no other starinterface having a higher priority has simultaneously sent a pilotsignal from the network node assigned to this other star interface. Astar interface transfers data from the assigned network node to theother star interfaces only when the send control signal is activated.

Patent claim 4 indicates that each star interface includes a first and asecond switch element. The switch elements as described in claim 5,which elements may be a switchable amplifier, control the flow ofmessages in a star interface in dependence on the pilot signal.

The decision which star interface is allowed to send data in the eventof an access contention is made by a decision circuit. Two embodimentsof decision circuits are explained in claims 6, 7 and 8.

Furthermore, the invention relates to a star node in a network forcoupling a plurality of network nodes.

Examples of embodiment of the invention will be further explainedhereinafter with reference to the Figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network in a star structure comprising a plurality ofnetwork nodes which are coupled via an active star node,

FIG. 2 shows an embodiment of a star interface in a star node,

FIG. 3 shows an embodiment of a star node with a plurality of starinterfaces,

FIG. 4 shows part of a network node with a pilot signal generator,

FIG. 5 shows part of a timing diagram of two signals in the network, and

FIG. 6 shows a further embodiment of a star interface.

DETAILED DESCRIPTION

An example of embodiment of the network according to the invention isshown in FIG. 1. This network comprises, for example, four network nodes1 to 4 which are coupled to each other via an active star node 9 through(twisted) line pairs 5 to 8 provided for a symmetrical signaltransmission. The active star node 9 performs a line adaptation, so thatthe line pairs (5 to 8) in the active star node 9 are terminated by thewave resistance and detects a pilot signal transmitted by a network node1 to 4. If the line pairs (5 to 8) were interconnected without an activestar node 9, there would be a mismatch for each line pair in the starnode as a result of the impedance jump from Z₀ to ⅓ Z₀, which is causedby the parallel combination of the respective other line pairs. Forgenerating a pilot signal, each network node 1 to 4 includes a pilotsignal generator.

The active star node 9 includes for each line pair (5 to 8) a starinterface, which enables the messages of a transmitting network node tobe transferred to all the other network nodes connected to the activestar. An example of embodiment of such a star interface is shown in FIG.2, which includes a line connector 10, a local pilot signal evaluationcircuit 11 and a local amplifier control circuit 12.

The line connector circuit 10 includes two switchable amplifiers 13 and14, a terminating impedance 15 and a pilot signal detector 16. A linepair (5 to 8) (see FIG. 1) is connected to the inputs of the switchableamplifier 13, to the outputs of the switchable amplifier 14, to theinputs of the pilot signal detector 16 and to the terminating impedance15. The value of the terminating impedance 15 corresponds to thecharacteristic impedance and is therefore used for the correct linetermination.

When the switchable amplifier 13 is activated by a switch signal rev_enproduced by the local amplifier control circuit 12, the data coming fromthe assigned line pair are led to a data line 17 (data) and thentransferred to the other star interfaces. When the switchable amplifier14 is activated by a switch signal drv_en, which is also produced by theamplifier control circuit 12, the data arriving over the data line 17from another star interface are fed to the assigned line pair (5 to 8).A switchable amplifier 13 or 14 may also be arranged as a seriescombination of amplifier and switch (switch element). In the closedstate of this switch the output signal of the amplifier 13 or 14 istransferred.

When the pilot signal detector 16 detects a pilot signal on the linepair (5 to 8), this indicates the control signal pt_det generated by thepilot signal detector, for example, by a change of state from a low to ahigh state. The output of the pilot signal detector 16 is connected tothe input of an inverting AND gate 18, which forms part of the pilotsignal evaluation circuit 11. The pilot signal evaluation circuit 11furthermore includes a memory arrangement 19 (for example, RS flip-flop)which can be set and reset and a further inverting AND gate 20, whichhas an open collector output. The memory arrangement 19 has a set inputS, a reset input R and an output Q. The truth table for this memoryarrangement 19 has the following contents:

S R Q 0 0 1 0 1 1 1 0 0 1 1 Q

The inverting output of the AND gate 18 is connected to the set input Sof the memory arrangement 19, whose output Q is connected to an input ofthe AND gate 20. The reset input R of the memory arrangement 19 is stillcoupled to the output of the pilot signal detector 16 and the output ofthe AND gate 20 is connected to a further input of the AND gate 18.

The inverting output of the AND gate 20, which AND gate produces a sendcontrol signal en_trx, is coupled to a voltage source via a resistor 21.The output of the inverting AND gate 20 thus represents an opencollector output.

The recognition of a pilot signal by the pilot signal detector 16 is notsufficient for determining whether the pilot signal has been sent by theassigned network node or whether the pilot signal has been fed to theassigned line pair via the star node 9. However, this is necessary fordetermining whether the assigned network node is an active sender of apilot signal and thus also of the subsequent data. For this purpose isused the pilot signal evaluation circuit 11 with the memory arrangement19 which stores the respective state of the control signal pt_det of thepilot signal detector.

The output signal on the output Q of the memory arrangement 19 controlsthe AND gate 20, which influences the send control signal en_trx and,together with the resistor 21 and the AND gates 20 of the other starinterfaces, forms a wired OR circuit. The send control signal en_trx isconsidered to be non-active as long as a high state (logic 1 level) isobtained. This is obtained by the connected resistor 21, which keeps thesend control signal en_trx at the logic “1” level. An activation takesplace i.e. the send control signal en_trx is active as a result of theswitching at the logic “0” level (active low). When one of the starinterfaces connected to the resistor 21 signals a send request by achange of state of the output signal of the assigned AND gate 20, thesend control signal en_trx becomes active and this change of state cantherefore be recognized by the further star interfaces.

If a star interface activates the send control signal en_trx through itsAND gate 20, this provides in all the other star interfaces that a sendrequest arriving at a later time, signaled by the control signal pt_det,cannot be met. This is ensured by the inverting AND gate 18, whichprevents the transfer of the control signal pt_det in the event of anactive send control signal en_trx. In the star interface that hasactivated the send control signal en_trx itself, the set signal for thememory arrangement 19 is also switched off by the AND gate 18. However,the previously set state is maintained by the memory arrangement untilthe assigned pilot signal detector 16 signals the end of thetransmission. At the end of the transmission the pilot signal detector16 deactivates the control signal pt_det and thereby resets the memoryarrangement 19 through the reset input R. Via the AND gate 20 the sendcontrol signal en_trx is no longer activated and brought back to theinactive state (logic 1 level) through the resistor 21.

In the active star node 9 is further included a decision circuit 22(FIG. 3). The decision circuit 22 has a decision element 23 for eachstar interface, which decision element 23 includes an OR gate 24. Asshown in FIG. 3, the decision elements 23 are chained and form thedecision circuit 22. Additionally, FIG. 3 shows the line connectors 10which are assigned to a decision element 23, local pilot signalevaluation circuits 11 and amplifier control circuits 12. A decisionelement 23 assigned to a star interface receives as a local send requestsignal p_det_local the output signal of the memory arrangement 19. Thelocal send request signal p_det_local is applied to a first input and adecision control signal trx_blk_in is applied to a second input of theOR gate 24. The output signal trx_blk_out of this OR gate 24 is appliedto the next decision element 23 as a decision control signal trx_blk_in.The first input of the OR gate 24 of the first decision element 23 ofthe chain is connected to a ground terminal (logic 0 level). Thiscircuit provides that the star interface, whose assigned decisionelement 23 is connected to ground, has the highest priority. The starinterfaces of the assigned subsequent decision elements 23 in the chainhave a lower priority depending on their position in the chain. The starinterface, whose decision element 23 is arranged at the end of thechain, consequently has the lowest priority (compare FIG. 3).

The output signal trx_blk_in of a decision element 23, which is assignedto the higher priority star interface, controls the switchableamplifiers 13 and 14 via the amplifier control circuit 12. The amplifiercontrol circuit 12 includes two OR gates 25 and 26, one inverter 27 andone AND gate 28. A first input of the OR gate 25 is connected to theoutput of the AND gate 20 and a second input of the OR gate 25 to theoutput of the memory arrangement 19. An inverting output of the OR gate25 leads to a first input of the OR gate 26, whose second input issupplied with the decision control signal trx_blk_in and whose output isconnected to a switch input (switch signal drv_en) of the switchableamplifier 14. The inverter 27 receives the decision control signaltrx_blk_in. The first input of the AND gate 28 is connected to theoutput of the inverter 27, the second input to the output of the memoryarrangement 19 and the output of the AND gate 28 to the switch input(switch signal rcv_en) of the switchable amplifier 13. The output of theinverter 27 is furthermore coupled to a second input of the AND gate 20.

In the following the cases are described when no star interface having ahigher priority signals a send request and thus the decision controlsignal trx_blk_in is not activated.

In the first case, it is assumed that the local send request signalpt_det_local is not active. This means that the memory arrangement 19 isnot set and no pilot signal was detected on the assigned line pairs (5to 8). Furthermore, the send control signal en_trx is not active either,which means that no other star interface signals the recognition of apilot signal either. Then the switch signal drv_en is deactivated forthe switchable amplifier 14 via the OR gates 25 and 26, which means thatthe amplifier 14 is switched off. At the same time also the switchableamplifier 13 is switched off via the switch signal rcv_en, which isguaranteed by the gates 27 and 28. This case is referred to as a stateof rest of the active star node 9.

In the second case, another star interface controls the local amplifiercontrol circuit 12 of the star interface concerned with the aid of thesend control signal en_trx. Via the activated send control signal en_trxthe switchable amplifier 14 is switched on by the switch signal drv_en,and the switchable amplifier 13 is switched off via the switch signalrcv_en. In this configuration the star interface conveys the data, whichis fed to the star node 9 via the data line 17, to the connected memoryarrangement 19. In order that on the assigned line pair (5 to 8) thepilot signal co-transmitted with the data does not lead to a feedback inthe amplifier control circuit 12, the control signal pt_det generated bythe pilot signal detector 16 is blocked at the AND gate 18. The sendcontrol signal en_trx provides that the control signal pt_det does nothave any influence on the set input S of the memory arrangement 19.

In the third case it is assumed that a star interface is the first oneto detect a pilot signal on the line pair (5 to 8). Then, via the ANDgate 18 and the memory arrangement 19, the local send request signalpt_det_local is activated and so is the send control signal en_trx viathe AND gate 20. As a consequence of the activation of the local sendrequest signal pt_det_local, the switchable amplifier 13 is turned onand the switchable amplifier 14 is turned off. Data transmitted by theconnected network node are then fed to the star node 9 and a feedback tothe line pair (5 to 8) is prevented by the switchable amplifier 14. Allthe other star interfaces behave as described above as a result of theactivated send control signal en_trx. Data arriving over the data line17 are transferred to the assigned line pair (5 to 8) via a switchableamplifier 14 of another star interface.

In the active star node 9 without a decision circuit 22, both in theevent of an error, which means that a network node sends outside thetime slot assigned to it, and during the start of the system, it ispossible that a simultaneous access of a plurality of star interfaces tothe data line 17 occurs. This means that at the same time two or moredifferent network nodes can transfer their data to the active star node9. How the network according to the invention solves such a media accesscontention is described in the following.

In the case of a media access contention in a star interface thedetection of a pilot signal has already led to the fact that the memoryarrangement 19 is set via the control signal pt_det and thus the localsend request signal pt_det_local is active. In parallel therewith, oneor more other star interfaces of the same star node 9 have also detecteda pilot signal on their assigned line pair and have also activated theirlocal send request signal pt_det_local. In this case the send controlsignal en_trx is activated by a plurality of star interfaces, withoutthis, with the aid of the AND gate 18, having any influence on thecontents of the memory arrangement 19. The activation of the sendcontrol signal en_trx by another star interface cannot be recognized bya star interface with its own send request (pilot signal occurs),because this is not made possible by the wired OR circuit describedabove. All the star interfaces having the intention to send and having adetected pilot signal therefore activate their respective switchableamplifier 13 and turn off the switchable amplifier 14. The result is acollision of the various data on the data line 17. As a result of theturned-off amplifier 14, this collision, however, does not becomevisible in the respective network nodes, which send their data to thenetwork node 9.

A priority control, which will be described in the following, thenovertakes the solution of this undesired network constellation. Aplurality of star interfaces activate, as described, their local sendrequest signal pt_det_local. These local send request signalspt_det_local are fed to the respectively assigned decision elements 23(compare FIG. 3). Only for the star interface having the highestpriority will the decision control signal trx_blk_in remain inactive andthus the set amplifier configuration will be maintained in this starinterface. For all the other star interfaces the decision control signaltrx_blk_in becomes active and effects a change in the amplifier controlcircuit 12. Via the OR gates 25 and 26 the switchable amplifier 14 isturned on and the switchable amplifier 13 is turned off. In addition,the decision control signal trx_blk_in avoids that a star interface,whose send request is not continued, furthermore activates the sendcontrol signal en_trx. The inverting AND gate 20 combines the local sendrequest signal pt_det_local and the decision control signal ntrx_blk_inapplied via the inverter 27 and thereby causes the local send requestsignal pt_det_local to have no influence anymore on the send controlsignal en_trx when the decision control signal trx_blk_in is activated.

The contention of a plurality of data meeting each other on the dataline 17 inside the star node 9 is taken out of the star node 9 andtransferred to the line pair (5 to 8) connected to the respectivetransmitting network node. On the line pairs (5 to 8) assigned to atransmitting network node there is then a contention between the data ofthe transmitting network node, whose assigned star interface has thehighest priority of the star interfaces causing the contention, and thedata of the respective network node connected to the line pair. Themedia access contention in a network having active star nodes 9 can berecognized by each transmitting network node. This network node is thenput to a position to stop its own send operation in case of acontention.

For the undisturbed functioning of the active star node 9 it isnecessary that in a certain dead time no network node is active or sendsmessages or data, respectively. In this state the star node is fullyblocked (all the amplifiers 13 and 14 are deactivated). In this state astar interface in the network node 9 waits for the new pilot signal bywhich a request for transmission of data is indicated.

Basically, the pilot signal must always be transmitted prior to thebeginning of the actual transmission of the message. Only then isensured that the active star node 9 in a contention-free communicationcontrol (TDMA) is timely configured and also the beginning of themessage reaches all the other network nodes.

FIG. 4 shows in what way the pilot signal is generated in a network node1 to 4 and transmitted over a line pair 5 to 8. If a network node wishesto send a data to other network nodes, a pilot signal generator 29 willreceive, for example, a start signal over a line 30. The pilot signalgenerator 29 then applies a pilot signal to a multiplexer 31, to whichare additionally sent data over a line 32. The signal produced by themultiplexer 31 is fed to the assigned line pair via an amplifier 33. Asignal coming from another network node is led from the line pair (5 to8) over an amplifier 34 to a line 35 to be further processed.

The multiplex shown in FIG. 4 may then be arranged both as atime-division multiplexer (sending the pilot signal as a start and stopsignal before and after the actual message, respectively) or as afrequency-division multiplexer. This means that the pilot signal canaccompany as a continuous signal the whole message to be transmitted, orthat it can be sent in the form of a start and stop signal. For example,by different durations it can be ensured that start and stop signals aresufficiently different and the change between transmission interval andtransmission pause is not mixed up.

The pilot signal may be generated in various ways. One possibility isthat it may be a periodic signal, whose frequency range lies outside thefrequency range utilized for the transmission of the messages. Thisfrequency range may lie above or below the useful frequency band, butalso in “gaps” of the useful frequency band, when the useful band isaccordingly specified due to the narrow-band nature of the pilot signal.A further possibility is to transmit the actual message as a symmetricalpush-pull signal and the pilot signal as an in-phase signal. Thein-phase signal may be embodied both in the form of a constant voltageand in the form of a periodic signal. A third possibility for the pilotsignal is that this pilot signal is realized in the form of specialsymbols preceding or following the transmission of the message.

The proposed invention is particularly suitable for use in networksfunctioning in accordance with the TTP protocol for a real-timecommunication, for example, in the motor vehicle (compare ElektronikHeft 14/1999: “TTP: “Drive by Wire” in greifbarer Nähe”, pp. 36 to 43).In this protocol is determined, on the one hand, when what transmitteris allowed to transmit with the aid of the contention-free TDMA accessmethod (TDMA=Time-Division Multiple Access), and on the other hand, adead time (interframe gap) is defined in which no transmitter is allowedto transmit. This mechanism guarantees that the active star node 9always comes back to the state of rest. The TDMA method guarantees thatalways only one network node is allowed to send a message at thepredefined time, and for this purpose, by means of its transmitted pilotsignal, activates in the star node, or causes the star interfaceassigned thereto, to transfer the messages. The resolution of mediaaccess contentions, which may occur also in the described protocolduring the starting phase of the system, is guaranteed by the prioritycontroller.

An additional advantage of the present invention, when applied to a TTPnetwork, consists in that for driving the so-called bus guardian acontrol signal is to be available in the network node, which controlsignal lies shortly before the beginning of the transmission of themessage. This control signal can immediately be used for driving thepilot signal generator 29 in that this control signal is applied to thepilot signal generator 29 over the line 30. Naturally, this controlsignal may also be generated with a respective leader routine, but alsoby the instance that itself initiates a send operation in the networknode.

In FIG. 5 this control signal is referred to as BG and the actual dataas data. The control signal BG is, for example, in a low state duringthe transmission of data. During this low state of the control signalthe data are to be transmitted. A first time space T1 after a change ofthe control signal to the low state and a second time space T2 before achange of the control signal to the high state are then to be selectedsuch that the active star node 9 is configured correctly and remainslike that to be able to transmit a message without any errors. It shouldfurther be observed that the TTP protocol supports different (constant)message delays between various network nodes in the network. In this waythe delay caused by the active star node 9 does not violate the TTPprotocol.

The network according to the invention enables the transmission of apilot signal with any kind of signal transmission for the messages froma network node 1 to 4. For example, a symmetrical push-pulltransmission, single line transmission or carrier frequency modulatedtransmission may be selected for the transmission of messages. With anin-phase coupling of the line pairs 5 to 8, also a supply voltage couldbe co-transmitted with the transmission of messages, as the case may be.

A further example of embodiment of a star interface is shown in FIG. 6,with the line connector circuit 10, a local pilot signal evaluationcircuit 36 and a local amplifier control circuit 37. The output of thepilot signal detector 16 producing the control signal pt_det isconnected to a first input of an inverting AND gate 38, which forms acomponent part of the pilot signal evaluation circuit 36 as do a memoryarrangement 39, an AND gate 40 and an OR gate 41. The inverting outputof the AND gate 38 is connected to the set input S of the memoryarrangement 39, whose reset input R is connected to the output of thepilot signal detector 16 and its output Q to a first input of the ANDgate 40. The memory arrangement 19 has the same truth table as thememory arrangement 19 of FIG. 2. The second input of the AND gate 40 andthe second input of the inverting AND gate 38 are connected to theoutput of the OR gate 41.

The amplifier control circuit 37 comprises an AND gate 42, an invertingOR gate 43 and an inverter 44. The output of the AND gate 42 producesthe switch signal rcv_en for the switch input of the switchableamplifier 13 and the output of the OR gate 43 produces the switch signaldrv_en for the switch input of the switchable amplifier 14. The firstinput of the OR gate 43 is connected both to the first input of the ANDgate 42 and to the first input of the OR gate 41 and the second input ofthe OR gate 43 to the input of the inverter 44 of both to the input ofthe inverter 44 and to the second input of the OR gate 41. The output ofthe inverter 44 further has a connection to the second input of the ANDgate 42.

The active star node includes, in addition to a multitude of starinterfaces, a decision circuit 45 with a decision decoder 46 and a1-from-m decoder 47. In the FIG. 2 the decision circuit 45 is arrangedfor three star interfaces. This means that the decision decoder 46 hasthree inputs 48 to 50 and the 1-from-m decoder 47 has four outputs 51 to54 (m=4). The decision decoder 46 and the 1-from-m decoder 47 arecombined via two connections (Y0, Y1). The input 48 of the decisiondecoder 46 is supplied with the signal trx_req2 from the output of theAND gate 40 of the star interface shown in FIG. 6 and the inputs 49 and50 with the signals trx_req1 and trx_req0 from outputs of the AND gate40 of further star interfaces not shown here. The output 51 of the1-from-m decoder 47 applies the signal en_trx2 to the first input of theOR gate 41 and the outputs 52 and 53 the signals en_trx1 and en_trx0 tothe first inputs of the OR gate 41 of the other star interfaces. Asignal star_idle of the output 54 of the 1-from-m decoder 47 is furtherapplied to every second input of the OR gate 41 of the star surfaces.

The decoder 46 may be described via the following truth table

trx_req2 trx_req1 trx_req0 Y1 Y0 0 0 0 0 0 0 0 1 0 1 0 1 x 1 0 1 X x 1 1and the 1-from-m decoder with m=4 via the following truth table

Y1 Y0 en_trx2 en_trx1 en_trx0 star_idle 0 0 0 0 0 1 0 1 0 0 1 0 1 0 0 10 0 1 1 1 0 0 0The outputs of the decision decoder 46 and the inputs of the 1-from-mdecoder 47, respectively, are referred to as Y0. “x” is understood tomean that the state may be any state.

It is assumed that the active star node 9 is in the rest state i.e. noneof the network nodes connected to the star node 9 sends data. If anetwork node wishes to send data, this is shown by the network node bysending a pilot signal. If the pilot signal detector 16 of the assignedstart interface detects the pilot signal, this is announced to thedecision decoder 46 on the output 48 by activation of the send controlsignal trx_req2 generated by the local pilot signal evaluation circuit36. The other inputs 49 and 50 of the decision decoder 46 receive thesend control signals trx_req1 and trx_req0, which come from the pilotsignal evaluation circuits 36 of the other star interfaces.

The decision decoder 46 and the following 1-from-m decoder 47 arearranged such that the decision control signal star_idle on the output54 of the 1-from-m decoder 47 is activated when none of the send controlsignals trx_req0, trx_req1 and trx_req2 produced by the respectivedecision decoders 46 is activated. In this state a send control signaltrx_req0, trx_req1 and trx_req2 may be activated when the associatedpilot signal detector 16 has detected the pilot signal. The decisioncircuit 45 is arranged so that the star interface that generates thesend control signal trx_req2 has a higher priority than the starinterface that generates the send control signal trx_req1 and than thestar interface that generates the send control signal trx_req0. The starinterface that generates the send control signal trx_req1 again has ahigher priority than the star interface that generates the send controlsignal trx_req0.

If a star interface activates the send control signal trx_req0 earlierthan the other star interfaces detect possible pilot signals, the othersend control signals trx_req1 and trx_req2 are prevented via the OR gate41 and the inverting AND gate 38 from being activated. The same holdsfor the other send control signals trx_req1 and trx_req2. This meansthat an activated decision control signal trx_req0, trx_req1 or trx_req2prevents the respective other send control signals formed by a pilotsignal evaluation circuit 36 from being activated.

The amplifier control circuit 37 is also controlled by the decisioncontrol signals star_idle and en_trx2, which are generated by the1-from-m decoder 47. The other two star interfaces not shown in FIG. 6receive the decision control signals star_idle and en_trx0 or en_trx1,respectively, for controlling their respective amplifier controlcircuits 37. In the state of rest, the two switchable amplifiers 13 and14 are turned off. When a pilot signal is detected by the assigned pilotsignal detector 16, the amplifier 14 and the amplifier 13 are turned offby an activation of the decision control signal en_trx2. In this mannerthe assigned network node (1 to 4) can transfer data via the data line17 to other star interfaces.

If a star interface has not detected a pilot signal, or if thegeneration of a send control signal of the pilot signal evaluationcircuit 36 has been blocked, the two decision control signals star_idleand en_trx2 (or en_trx0 or en_trx1), generated by the 1-from-m decoder47 and applied to the assigned star interface, are not activated. Theamplifier control circuit 37 then activates the switchable amplifier 14to receive data from the control line 17. The amplifier 13 is turnedoff.

The media access contention is resolved by the decision circuit 45 inthe example of embodiment of FIG. 6. Only the star interface having thehighest priority can send data in the event of a simultaneous sendrequest (via the pilot signal). The star interfaces having a lowerpriority receive non-activated decision control signals from the1-from-m decoder 47 to prevent the associated amplifier 13 from beingstarted. The other amplifier 14, on the other hand, is turned on.

1. A network comprising: a plurality of network nodes, the network nodesare directly coupled to each other via at least one star node; the atleast one star node includes a plurality of star interfaces that areassigned to at least one respective network node, with one starinterface transferring data, in dependence on a pilot signal varyingfrequency, from the assigned respective network node to the other starinterfaces or from another star interface to the assigned respectivenetwork node, pilot signal evaluation circuit generates a send controlsignal and activates the send control signal if a pilot signal has beensent by the assigned network node and no other star interface having ahigher priority has simultaneously sent a pilot signal from the networknode assigned to this other star interface, a star interlace is providedfor transferring data from the assigned network node to the other starinterfaces only when the send control signal is activated, each starinterface includes a first and second switching amplifiers, the firstswitching amplifier in an activated state passes data from the assignednetwork node to the other star interfaces, and the second switchingamplifier in an activated state passes data from the other starinterfaces to the assigned network node, and in the event of receivingthe active send control signal, the first switching amplifier is in anactive state and the second switching amplifier is in a non-activestate, a decision circuit evaluates the send control signals of all thestar interfaces, and with a simultaneous occurrence of various sendcontrol signals corresponding to the simultaneous arrival of at leasttwo pilot signals at respective star interfaces, the decision circuitreleases via a decision control signal a certain star interface for thetransmission of data, and the decision circuit includes a chain ofin-line decision elements each having an OR gate that combines theoutput signal of the previous decision element with a local send requestsignal generated by the pilot signal evaluation circuit and indicatingthe presence of the pilot signal, the output signal of an OR gate beingthe decision control signal for the star interface assigned to the nextdecision element in the chain.
 2. A network as defined in claim 1,wherein the decision circuit includes a decision decoder decoding thesend control signals, and a 1-from-m decoder receiving the outputsignals of the decision decoder, which 1-from-m decoder generates arespective decision control signal for the respective star interfaces.